Reluctance motors are doubly salient, that is, they have tooth-like poles on both the stator and the rotor. In addition, there are windings on the stator but no windings on the rotor of reluctance motors. Each pair of diametrically opposite stator windings is connected in series to form one phase of the motor.
Torque is produced in variable reluctance motors by switching current on in the stator phases in a predetermined sequence so that a magnetic force of attraction results between the rotor and stator poles as they approach each other. The current is switched off in each pair of windings at the commutation point before the rotor poles nearest the stator poles of that pair of windings rotate past the aligned position, otherwise the magnetic force of attraction will produce a negative or braking torque.
The torque developed is independent of current direction. Therefore, unidirectional current pulses synchronized with rotor movement can be generated in a converter using a single unidirectional current switching element such as a thyristor or transistor in each leg of the converter, and supplied to the corresponding phase of the motor.
Each time a phase of the motor is energized by closing a switch in the converter, current flows in the pair of stator windings of that phase, providing energy from a DC supply to the motor. The energy drawn from the supply is converted partly into mechanical energy, by causing the rotor to rotate towards a minimum reluctance configuration, and converted partly into stored energy in the magnetic field. When the switch is opened, the stored magnetic energy is preferably returned to the DC supply.
The motor may be run open-loop as in a variable reluctance stepping motor, or may be run closed-loop as in a switched reluctance motor. In addition, the motor may be operated such that no two phases are conducting simultaneously (i.e. nonoverlapping conduction intervals) or such that some phases do conduct simultaneously (i.e. overlapping conduction intervals). Furthermore, the same machine may be operated as a generator by driving the rotor and changing the timing of the switches in the converter with respect to the rotor position.
Reluctance rotors are very attractive candidates for high operating speeds due to their simple rotor structure. The active portion of the rotor is normally constructed using only magnetic laminations as no windings or permanent magnets are necessary. Such rotors incur high windage losses as a result of their salient structure. In addition, in order to operate at high speeds, the rotor will incur internal forces close to the stress limits of the lamination material. These stresses come directly from centrifugal forces and indirectly from the shrink fit required to prevent a loss of mechanical contact between the laminations and the shaft at high speeds.
It has been proposed that salient pole motors could be strengthened, while reducing the windage losses, by fabricating the salient pole rotor laminations with segments of a nonmagnetic material (e.g., stainless steel) between the rotor poles. This method of fabricating salient pole rotor laminations is specifically described in G. B. Kliman U.S. application Ser. No. 138,404 filed Dec. 28, 1987, now U.S. Pat. No. 4,916,346 and assigned to the instant assignee, which is hereby specifically incorporated by reference.